Optical communications using multiplexed single sideband transmission and heterodyne detection

ABSTRACT

A transmitter subsystem generates an optical signal which contains multiple subbands of information. The subbands have different polarizations. For example, in one approach, two or more optical transmitters generate optical signals which have different polarizations. An optical combiner optically combines the optical signals into a composite optical signal for transmission across an optical fiber. In another approach, a single optical transmitter generates an optical signal with multiple subbands. The polarization of the subbands is varied, for example by using a birefringent crystal. In another aspect of the invention, each optical transmitter generates an optical signal containing both a lower optical sideband and an upper optical sideband (i.e., a double sideband optical signal). An optical filter selects the upper optical sideband of one optical signal and the lower optical sideband of another optical signal to produce a composite optical signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 09/728,373, “Optical Communications System UsingHeterodyne Detection”, by Ting K. Yee and Peter H. Chang, filed Nov. 28,2000, which is a continuation-in-part of pending U.S. patent applicationSer. No. 09/474,659, “Optical Communications System Using HeterodyneDetection”, by Ting K. Yee and Peter H. Chang, filed Dec. 29, 1999.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/265,251, “Fiber Optic Communications UsingOptical Single Sideband Transmission Including using Interleaver Filtersand Heterodyne Detection and Apparatus for Impairment Compensation usingNonlinear Phase Conjugation,” by Ting K. Yee, et al., filed Jan. 30,2001.

This application relates to pending U.S. patent application Ser. No.09/746,261, “Wavelength-Locking of Optical Sources,” by Shin-ShengTarng, et al., filed Dec. 20, 2000.

This application also relates to pending U.S. patent application Ser.No. 09/747,261, “Fiber Optic Communications using Optical SingleSideband Transmission and Direct Detection,” by Ting K. Yee and Peter H.Chang, filed Dec. 20, 2000.

This application also relates to pending U.S. patent application Ser.No. 09/854,153, “Channel Gain Control For An Optical CommunicationsSystem Utilizing Frequency Division Multiplexing,” by Laurence J. Newelland James F. Coward, filed May 11, 2001; and pending U.S. patentapplication Ser. No. 09/569,761, “Channel Gain Control For An OpticalCommunications System Utilizing Frequency Division Multiplexing,” byLaurence J. Newell and James F. Coward, filed May 12, 2000.

This application also relates to pending U.S. patent application Ser.No. 09/405,367, “Optical Communications Networks Utilizing FrequencyDivision Multiplexing,” by Michael W. Rowan, et al., filed Sep. 24,1999; which is a continuation-in-part of pending U.S. patent applicationSer. No. 09/372,143, “Optical Communications Utilizing FrequencyDivision Multiplexing and Wavelength-Division Multiplexing,” by Peter H.Chang, et al., filed Aug. 20, 1999; which is a continuation-in-part ofU.S. patent application Ser. No. 09/229,594, “Electrical Add-DropMultiplexing for Optical Communications Networks Utilizing FrequencyDivision Multiplexing,” by David B. Upham, et al., filed Jan. 13, 1999;which is a continuation-in-part of U.S. patent application Ser. No.09/035,630, “System and Method for Spectrally Efficient Transmission ofDigital Data over Optical Fiber”, by Michael W. Rowan, et al., filedMar. 5, 1998.

The subject matter of all of the foregoing applications is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical fiber communications, andmore particularly, to the use of single sideband transmission andheterodyne detection for optical fiber communications systems.

2. Description of the Related Art

As the result of continuous advances in technology, particularly in thearea of networking, there is an increasing demand for communicationsbandwidth. For example, the growth of the Internet, home office usage,e-commerce and other broadband services is creating an ever-increasingdemand for communications bandwidth. Upcoming widespread deployment ofnew bandwidth-intensive services, such as xDSL, will only furtherintensify this demand. Moreover, as data-intensive applicationsproliferate and data rates for local area networks increase, businesseswill also demand higher speed connectivity to the wide area network(WAN) in order to support virtual private networks and high-speedInternet access. Enterprises that currently access the WAN through T1circuits will require DS-3, OC-3, or equivalent connections in the nearfuture. As a result, the networking infrastructure will be required toaccommodate greatly increased traffic.

Optical fiber is a transmission medium that is well-suited to meet thisincreasing demand. Optical fiber has an inherent bandwidth which is muchgreater than metal-based conductors, such as twisted pair or coaxialcable. There is a significant installed base of optical fibers andprotocols such as SONET have been developed for the transmission of dataover optical fibers. Typical communications system based on opticalfibers include a transmitter, an optical fiber, and a receiver. Thetransmitter converts the data to be communicated into an optical formand transmits the resulting optical signal across the optical fiber tothe receiver. The receiver recovers the original data from the receivedoptical signal. Recent advances in transmitter and receiver technologyhave also resulted in improvements, such as increased bandwidthutilization, lower cost systems, and more reliable service.

However, current optical fiber systems also suffer from drawbacks whichlimit their performance and/or utility. For example, optical fiberstypically exhibit dispersion, meaning that signals at differentfrequencies travel at different speeds along the fiber. Moreimportantly, if a signal is made up of components at differentfrequencies, the components travel at different speeds along the fiberand will arrive at the receiver at different times and/or with differentphase shifts. As a result, the components may not recombine correctly atthe receiver, thus distorting or degrading the original signal. In fact,at certain frequencies, the dispersive effect may result in destructiveinterference at the receiver, thus effectively preventing thetransmission of signals at these frequencies. Dispersion effects may becompensated by installing special devices along the fiber specificallyfor this purpose. However, the additional equipment results inadditional power loss (e.g., insertion loss) as well as in additionalcost, and different compensators will be required for different typesand lengths of fiber. Other fiber effects, such as fiber nonlinearities,can similarly degrade performance.

As another example, the transmitter in an optical fiber system typicallyincludes an optical source, such as a laser, and an external modulator,such as a Mach-Zender modulator (MZM). The source generates an opticalcarrier and the modulator is used to modulate the optical carrier withthe data to be communicated. In many applications, linear modulators arepreferred in order to increase the performance of the overall system.MZMs, however, are inherently nonlinear devices. Linear operation isapproximated by biasing the MZM at its quadrature point and thenlimiting operation of the MZM to a small range around the quadraturepoint, thus reducing the effect of the MZM's nonlinearities. However,this results in an optical signal with a large carrier (which containsno information) and a small modulated signal (which contains the data tobe communicated). A larger optical signal to noise ratio is required tocompensate for the large carrier.

As a final example, optical fibers have an inherently large bandwidthavailable for the transmission of data, but constructing transmittersand receivers which can take advantage of this large bandwidth can beproblematic. First, current approaches, such as the on-off keying andtime-division multiplexing of signals used in the SONET protocols,cannot be extended to higher speeds in a straightforward manner. This isbecause current electronics technology limits the speeds at which theseapproaches can be implemented and electronics fundamentally will nothave sufficient bandwidth to fill the capacity of a fiber. Even if thiswere not a limitation, current modulation schemes such as on-off keyingare not spectrally efficient; more data can be transmitted in lessbandwidth by using more efficient modulation schemes.

Current optics technology also prevents the full utilization of afiber's capacity. For example, in wavelength division multiplexing,signals are placed onto optical carriers of different wavelengths andall of these signals are transmitted across a common fiber. However, thecomponents which combine and separate the different wavelength signalscurrently place a lower limit on the spacing between wavelengths, thusplacing an upper limit on the number of wavelengths which may be used.This also leads to inefficient utilization of a fiber's bandwidth.

The ever-increasing demand for communications bandwidth furtheraggravates many of the problems mentioned above. In order to meet theincreasing demand, it is desirable to increase the data rate oftransmission across each fiber. However, this typically can only beachieved by either increasing the bandwidth being utilized and/or byincreasing the spectral efficiency of the encoding scheme. Increasingthe bandwidth, however, aggravates frequency-dependent effects, such asdispersion. Increasing the spectral efficiency increases the signal tonoise requirements.

Thus, there is a need for optical communications systems which morefully utilize the available bandwidth of optical fibers. There isfurther a need to reduce or eliminate the deleterious effects caused byfiber dispersion, to reduce the power contained in the optical carrier,and to combat the many drawbacks mentioned above.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical communicationssystem is for communicating information across an optical fiber andincludes a transmitter subsystem. The transmitter subsystem includes atleast two optical transmitters coupled to an optical combiner. Eachoptical transmitter generates an optical signal containing a subband ofinformation. The optical signals have different polarizations, whichpreferably are orthogonal polarizations. The optical combiner opticallycombines the optical signals into a composite optical signal.

In another aspect of the invention, the transmitter subsystem includesan optical transmitter coupled to a polarization controlling device. Theoptical transmitter generates an optical signal containing at least twosubbands of information. The polarization controlling device, forexample a birefringent crystal, varies a polarization of the subbands sothat the subbands have different polarizations.

The use of different polarizations yields many benefits. For example,subbands with different polarizations will interact less since they havedifferent polarizations. Thus, unwanted effects due to phenomena such asfour-wave mixing and cross-phase modulation will be reduced between thedifferently polarized subbands.

In another aspect of the invention, the transmitter subsystem includestwo optical transmitters, an optical combiner, and an optical filtercoupled in series. Each optical transmitter generates an optical signalcontaining both a lower optical sideband and an upper optical sideband(i.e., a double sideband optical signal). The optical combiner opticallycombines the two optical signals. The optical filter then selects theupper optical sideband of one optical signal and the lower opticalsideband of the other optical signal to produce a composite opticalsignal. In one embodiment, the optical filter includes two Bragg filterscoupled in series. In another aspect of the invention, the transmittersubsystem also includes a wavelength-locking device coupled to theoptical transmitters for locking a frequency separation of the opticalsignals to a predetermined value. In general, one advantage of thisapproach is that the two optical sidebands (and, hence, also thesubbands which they contain) can be more densely spaced in comparison towavelength division multiplexing approaches, thus resulting in higherbandwidth utilization.

In a preferred embodiment, each optical transmitter includes at leasttwo electrical transmitters, an FDM multiplexer and an E/O convertercoupled in series. Each electrical transmitter generates electricalchannels. The FDM multiplexer combines the electrical channels into anelectrical high-speed channel using FDM. The electrical high-speedchannel further includes a tone. The E/O converter converts theelectrical high-speed channel into the optical signal for the opticaltransmitter. In one specific implementation, there are two opticaltransmitters. One generates an optical signal containing at least twosubbands and a tone, each subband having a capacity of approximately 2.5Gbps of information (i.e., same data capacity as an OC-48 signal). Theother generates an orthogonally polarized optical signal containing atleast two other 2.5 Gbps subbands and a tone. An optical filter selectsthe upper optical sideband of one optical signal and the lower sidebandof the other optical signal. Thus, the total capacity for thetransmitter subsystem is sixteen 2.5 Gbps subbands, or approximately 40Gbps.

In another aspect of the invention, the optical communications systemalso includes a receiver subsystem coupled to the transmitter subsystemby an optical fiber. In a preferred embodiment for the case when thesubbands within the composite optical signal have differentpolarizations, the receiver subsystem includes a polarizing splittermodule coupled to a plurality of heterodyne receivers. The polarizingsplitter module splits the composite optical signal according topolarization, for example into its constituent subbands. The heterodynereceivers then recover the subbands.

In further accordance with the invention, a method for transmittinginformation across an optical fiber includes the following steps. Twooptical signals are generated. Each optical signal contains a subband ofinformation, but the two optical signals have different polarizations.They are optically combined into a composite optical signal, which istransmitted across an optical fiber.

Another method according to the invention includes the following steps.An optical signal containing at least two subbands of information isgenerated. The polarizations of the subbands are varied so that thesubbands have different polarizations. The optical signal is thentransmitted across an optical fiber.

Yet another method according to the invention includes the followingsteps. Two optical signals are generated. Each optical signal contains alower optical sideband and an upper optical sideband. The two opticalsignals are optically combined and then optically filtered. Thefiltering selects the lower optical sideband of one signal and the upperoptical sideband of the other signal. The resulting composite opticalsignal is transmitted across an optical fiber.

BRIEF DESCRIPTION OF THE DRAWING

The invention has other advantages and features which will be morereadily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawing, in which:

FIG. 1 is a block diagram of a system 100 illustrating one aspect of thepresent invention;

FIG. 2 is a flow diagram illustrating a method 200 for transmitting aninformation signal across a fiber using the system 100;

FIG. 3 is a diagram of one embodiment 310 of transmitter 110 using aMach-Zender modulator;

FIG. 4 is a graph illustrating a transfer function 400 for MZM 314;

FIG. 5 is a diagram of another embodiment 510 of transmitter 110 using athree-armed modulator;

FIG. 6 is a block diagram of one embodiment 690 of signal extractor 190based on squaring a signal containing a tone and a sideband;

FIG. 7 is a block diagram of another embodiment 790 of signal extractor190 based on multiplying a tone with a sideband;

FIG. 8 is a block diagram of yet another embodiment 890 of signalextractor 190 using separate extraction paths to process differentsidebands;

FIG. 9 is a block diagram of one embodiment 990 of signal extractor 890based on multiplying a tone with a sideband;

FIG. 10 is a diagram of another embodiment 1010 of transmitter 110 usingpilot tones;

FIG. 11 is a block diagram of a system 1100 illustrating another aspectof the invention;

FIG. 12 is a block diagram of a system 1200 illustrating yet anotheraspect of the invention;

FIGS. 13A-13E are graphs illustrating the spectra of various signals insystem 1200;

FIGS. 14A-14B are graphs illustrating the spectra of various signals insystem 1100;

FIGS. 15A-15D are graphs illustrating the spectra of various examplecomposite signals;

FIG. 16 is a block diagram of a system 1600 illustrating yet anotheraspect of the invention;

FIG. 17 is a block diagram of one embodiment of optical transmitter1610;

FIG. 18A is a block diagram of a wavelength locking device 1800 for usewith system 1600;

FIG. 18B is a graph illustrating the passband of optical filter 1615 asused in conjunction with wavelength locking device 1800;

FIG. 18C is a graph illustrating the passband of one implementation ofoptical filter 1615;

FIG. 19 is a block diagram of another embodiment of optical transmitter1610;

FIGS. 20A-20B are graphs illustrating the spectra and polarization ofvarious signals in optical transmitter 1900;

FIG. 21 is a block diagram of another embodiment of receiver subsystem1604;

FIG. 22 is a block diagram of another embodiment of optical transmitter1610;

FIG. 23 is a diagram showing the polarization of different subbandsresulting from optical transmitter 2200;

FIGS. 24A-25B are graphs illustrating various other spectra according tothe invention;

FIG. 26 is a graph illustrating the periodic pass bands of a combfilter; and

FIG. 27 is a block diagram of another embodiment of optical transmitter1610.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram of a system 100 illustrating one aspect of thepresent invention. System 100 includes a transmitter 110 coupled to areceiver 130 by optical fiber 120. The receiver 130 preferably includesa heterodyne detector 180 coupled to a signal extractor 190. System 100is used to transmit an information signal from transmitter 110 toreceiver 130 via fiber 120.

With reference to the flow diagram of FIG. 2 as well as to FIG. 1,system 100 operates as follows. The frequency spectrum of an exampleinformation signal is shown by spectrum 140, which is characterized by afrequency f_(S). The frequency f_(S) could be zero, for example, if theinformation signal is based on on-off keying. The information signal 140may be any of a variety of signals. For example, it may be a single highspeed data stream. Alternately, it may contain a number of data streamswhich are time-division multiplexed together, for example, if 64 OC-3data streams are combined together to form a single OC-192 signal, whichserves as the information signal 140. As another example, theinformation signal may include a number of constituent signals, each ofwhich occupies a different frequency band within spectrum 140. In otherwords, the constituent signals may be frequency division multiplexedtogether. Other types of information signals 140 and methods forcombining constituent signals to form the information signal 140 will beapparent.

Transmitter 110 receives 210 the information signal 140 and generates220 an optical signal 142. Optical signal 142 is characterized by acarrier frequency f_(C) and includes at least one sideband 144 based onthe information signal 140 and at least one tone 146, shown at afrequency f_(t) in the following examples. Various techniques may beused to achieve this function. In a preferred embodiment, transmitter110 includes an optical source 112 coupled to an optical modulator 114.Examples of optical sources include solid state lasers and semiconductorlasers. Example optical modulators 114 include Mach Zehnder modulators,electro-optic modulators, and electro-absorptive modulators. The opticalsource 112 produces an optical carrier at the carrier frequency f_(C).The modulator 114 receives 210 the information signal 140 and modulatesthe optical carrier with the information signal 140 to generate 220optical signal 142. In the example of FIG. 1, double sideband modulationis illustrated, resulting in two sidebands (upper sideband 144U andlower sideband 144L) which are centered about the carrier frequencyf_(C). Other types of modulation, such as single sideband modulation,could also be used. Continuing this example, the modulator 114 alsoproduces a significant signal at the carrier frequency f_(C), whichserves as a tone 146. Alternately, transmitter 110 may include aninternally modulated laser. In this case, the information signal 140drives the laser, the output of which is the optical signal 142.

The optical signal 142 is transmitted 230 over fiber 120 to receiver130. Current optical fibers have two spectral regions which are commonlyused for communications: the 1.3 and 1.55 micron regions. At awavelength of 1.3 micron, transmission of the optical signal isprimarily limited by attenuation in the fiber 120; dispersion is less ofa factor. Conversely, at a wavelength of 1.55 micron, the optical signalwill experience more dispersion but less attenuation. Hence, the opticalsignal preferably has a wavelength either in the 1.3 micron region orthe 1.55 micron region and, for long distance communications systems,the 1.55 micron region is generally preferred.

At receiver 130, heterodyne detector 180 receives 235 the incomingoptical signal 142 and also receives 240 an optical local oscillatorsignal 134 at a frequency f_(LO). In FIG. 1, the local oscillator signal134 is shown at a frequency f_(LO) which is lower than the carrierfrequency f_(c), but the local oscillator signal 134 may also be locatedat a frequency f_(LO) which is higher than the carrier frequency f_(c).Examples of optical local oscillators 132 include solid state lasers andsemiconductor lasers. The optical signal 142 and local oscillator signal134 are combined 245 and heterodyne detection 250 of the combined signaleffectively downshifts the optical signal 142 from a carrier atfrequency f_(c) to a frequency Δf, which is the difference between thelocal oscillator frequency f_(LO) and the carrier frequency f_(c). Theresulting electrical signal has spectrum 150. Note that both sidebands154L and 154U, and tone 156 have also been frequency downshiftedcompared to optical signal 142. Signal extractor 190 then mixes 260 atleast one of the sidebands 154 with one of the tones 156 to produce anumber of frequency components, including one frequency component 170located at the difference frequency Δf between the relevant sideband 154and tone 156. This difference component 170 contains the informationsignal 140, although it may be offset in frequency from the originalfrequency f_(s), depending on the frequencies of the sideband 154 andtone 156. Frequency components other than the difference component 170may be used to recover the information signal. For example, the mixing260 typically also produces a sum component located at the sum of thefrequencies of the relevant sideband 154 and tone 156, and theinformation signal 140 may be recovered from this sum component ratherthan the difference component. If more than one sideband 154 isprocessed by signal extractor 190, each sideband 154 is processedseparately from the others in a manner which prevents destructiveinterference between the sidebands.

However, recovering the information signal 140 based on the differencecomponent of sideband 154 and tone 156 is advantageous because itresults in noise cancellation. For example, sideband 154L and tone 156are affected similarly by laser phase noise produced by optical source112 and optical local oscillator 132. Using the difference componenteffectively subtracts the laser phase noise in sideband 154L from thelaser phase noise in tone 156, resulting in significant cancellation ofthis noise source. In contrast, using the sum component wouldeffectively reinforce the laser phase noise.

Processing the sidebands 154 separately from each other is alsoadvantageous because it significantly reduces dispersion effects causedby fiber 120. For example, in direct detection receivers, upper andlower sidebands 154U and 154L would be processed together and, atcertain frequencies for the sidebands 154 and lengths of fiber 120, thedispersion effects of fiber 120 would cause the two sidebands todestructively interfere, significantly impairing the recovery ofinformation signal 140. By processing sidebands 154 separately from eachother, signal extractor 190 avoids this deleterious dispersion effect.

In a preferred embodiment, heterodyne detector 180 includes a combiner136 and a square law detector 137 coupled in series. Combiner 136preferably is a fiber coupler, due to its low cost and applicability tofiber systems, although other types of combiners may be used. Square lawdetector 137 preferably is a PIN diode. Combiner 136 receives 235 theincoming optical signal 142 at one of its inputs and receives 240 theoptical local oscillator signal 134 at the other input. Combiner 136combines the local oscillator signal 134 with the optical signal 142 toproduce the combined signal with spectrum 160. Heterodyne detector mayalso include a polarization controller 139 coupled to the combiner 136for matching the polarizations of the optical signal 142 and the localoscillator signal 134 so that the two signals are mixed efficiently atthe square law detector 137.

In a preferred embodiment, the polarization controller 139 matches thepolarization of the local oscillator 134 to the polarization of the tone146. This matching is particularly advantageous when a polarizationtracking algorithm is used because the tone 146 is stable and does nothave substantial amplitude variation and therefore provides betterlocking of the polarizations. In fibers having measurable polarizationmode dispersion, after propagation through the fiber, each sideband 144and the tone 146 can have slightly different polarizations, thusresulting in attenuation of the detected electrical signal due to thepolarization mismatch. Generally, the further the separation infrequency between the sideband 144 and the tone 146, the stronger theattenuation of the detected electrical signal. This attenuation can bemitigated by boosting the transmit power of the affected subbands. Forexamples of methods for mitigating the attenuation of power in thesubbands of the detected electrical signals, including boosting thetransmit power of subbands, see co-pending U.S. patent application Ser.No. 09/854,153, “Channel Gain Control For An Optical CommunicationsSystem Utilizing Frequency Division Multiplexing,” by Laurence J. Newelland James F. Coward, filed May 11, 2001; and co-pending U.S. patentapplication Ser. No. 09/569,761, “Channel Gain Control For An OpticalCommunications System Utilizing Frequency Division Multiplexing,” byLaurence J. Newell and James F. Coward, filed May 12, 2000.

In FIG. 1, the polarization controller 139 is shown located between thelocal oscillator 132 and combiner 136 and controls the polarization ofthe local oscillator signal 134. Alternately, the polarizationcontroller 139 may be located between the fiber 120 and combiner 136 andcontrol the polarization of the optical signal 142. In another approach,polarization controller 139 may control the polarizations of bothsignals 134 and 142. Square law detector 137 produces a photocurrentwhich is proportional to the intensity of signal 160, which effectivelymixes together the various frequency components in spectrum 160. Theresulting electrical signal has a number of frequency components locatedat different frequencies, with the components of interest shown byspectrum 150. Spectrum 150 is similar to spectrum 142, but frequencydownshifted from the carrier frequency f_(C) to the difference frequencyΔf.

FIGS. 3-5 illustrate various embodiments of transmitter 110 and FIGS.6-9 are examples of various embodiments of signal extractor 190. Theseembodiments are illustrated using the example of FIG. 1 in which opticalsignal 142 includes two sidebands 144 and the optical carrier functionsas a tone 146. The invention, however, is not limited to this specificexample. Modulation schemes besides double sideband may be used (e.g.,single sideband). Similarly, the tone 146 may be located at frequenciesother than the carrier frequency f_(c) and/or multiple tones 146 may beused.

FIG. 3 is a diagram of one embodiment 310 of transmitter 110, in whichmodulator 114 includes a Mach-Zender modulator (MZM) 314. MZM 314includes two arms 316A and 316B, and an electrode 318 for receivinginformation signal 140. The optical carrier produced by source 112 isreceived by MZM 314, which splits it into two signals, one propagatingthrough each arm 316. The information signal 140 applied to electrode318 produces an electric field across each of the arms 316, causing adifference in the optical path through each arm 316 (e.g., due to theelectro-optic effect). As a result of this difference in optical path,the optical signals propagating through the two arms 316A and 316B willeither constructively or destructively interfere when they arerecombined at the output of the MZM 314. In other words, the output ofMZM 314 depends on the relative phase difference between the two arms316, which in turn depends on information signal 140.

FIG. 4 graphs the intensity of MZM 314 as a function of the relativephase shift between the two arms 316. Since MZM 314 is interferometric,the intensity of its optical output is a sinusoidal function of therelative phase shift between the two arms 316. For example, if therelative phase shift between the two arms 316 is a multiple of 2π, thenthe signals in the two arms will constructively interfere, yielding amaximum intensity at the output as indicated by points 402A-402C. At theother extreme, two arms 316 which are out of phase will destructivelyinterfere, yielding a minimum intensity at the output as shown by points404A-404B, which shall be referred to as V_(π) points. The interim casesresult in the raised cosine transfer function 400 of FIG. 4. Asdescribed above, the relative phase shift is determined by the receivedinformation signal 140.

In one aspect of transmitter 310, the MZM 314 is biased at one of thequadrature points 406A-406D. At these quadrature points 406, the raisedcosine transfer function may be used to approximates a linear transferfunction, particularly if the modulator 314 is operated over a limitedrange around the quadrature points 406. When operated in this fashion,transmitter 310 results in the optical signal shown in spectrum 320. Theraised cosine nature of transfer function 400 results in dual sidebands324L and 324U; and operation at the quadrature point 406 results in alarge signal at the carrier frequency f_(c), which may be used as a tone326.

FIG. 5 is a diagram of another embodiment 510 of transmitter 110, inwhich the optical modulator 114 includes a three-armed modulator 514.Modulator 514 includes three arms 516A-516C. Two arms 516A-516B form aconventional MZM and information signal 140 modulates the signal inthese two arms in the same manner as MZM 314 of FIG. 3. However, the MZMformed by arms 516A-516B is not biased at one of the quadrature points406. Rather, it is operated at one of the V_(π) points 404. The resultis an optical signal which includes two sidebands 524L and 524U but nooptical carrier at f_(C) since operation at the V_(π) point 404suppresses the carrier. The third arm 516C is used to reintroduce theoptical carrier, preferably in a controlled manner by adjusting both theamplitude and phase of the carrier. For example, the amplitude and phasecould be determined by adjusting the splitting ratios between the threearms 516 and/or the lengths of the arms 516, respectively.Alternatively, control signal 530 could be used to adjust elements inarm 516C which control the amplitude and phase of the carrier in thearm. This may be accomplished by using, for example, separate phase andamplitude modulation elements. The reintroduced carrier then functionsas a tone 526 in optical signal 520. This approach is advantageouscompared to transmitter 314 because the amplitude and phase of opticalcarrier 526 may be tailored for different purposes. For example, sinceoptical carriers 526 and 326 do not carry any information, the amplitudeof carrier 526 may be minimized to reduce wasted power whereas theamplitude of carrier 326 is fixed by quadrature point 406.

A similar result may be obtained by various other approaches. Forexample, the third arm 516C may be replaced by an optical fiber. Some ofthe optical carrier produced by source 112 is diverted to the opticalfiber and then recombined with the output produced by the MZM formed byarms 516A-516B. In another approach, the MZM formed by arms 516A-516Bmay be biased at a point other than the V_(π) point 404, thus producingan optical carrier. However, the phase and/or amplitude of theunmodulated carrier in arm 516C may be adjusted so that it interfereswith the carrier produced by arms 516A-516B to generate an opticalcarrier with a desirable amplitude. The net result is an optical carrierof reduced amplitude. Alternately, referring again to FIG. 3, MZM 314may be biased at a point close to but slightly offset from the V_(π)points 404. The slight offset will result in some carrier beingintroduced into the optical signal, thus resulting in a spectrum 330with a reduced optical carrier as in spectrum 520.

FIG. 6 is a block diagram of one embodiment 690 of signal extractor 190based on squaring a signal containing a tone and a sideband. Signalextractor 690 includes a bandpass filter 610, a square law device 620,and a low pass filter 630 coupled in series. The filters 610, 630 may beimplemented in many different ways, for example, by a DSP chip or otherlogic device implementing a digital filter, a lump LC filter, a surfaceacoustic wave filter, a crystal-based filter, a cavity filter, or adielectric filter. Other implementations will be apparent. The squarelaw device 620 also may be implemented in many different ways. A diodeis one common implementation.

Signal extractor 690 recovers the information signal 140 from electricalsignal 150 as follows. Bandpass filter 610 frequency filters one of thesidebands and one of the tones from electrical signal 150. In thisexample, signal 150 includes two sidebands 154 and an optical carrier156. Bandpass filter 610 passes the upper sideband 154U and the opticalcarrier 156, and blocks the lower sideband 154L, thus producing spectrum660. The square law device 620 squares the filtered components 660,resulting in spectrum 670. Spectrum 670 includes frequency components672 located at the difference of frequencies between sideband 154U andtone 156, and also frequency components 674 located at the sum of thesefrequencies. Low pass filter 630 selects the difference components 672,thus recovering the information signal 140.

As noted previously, selection of the difference components 672 ratherthan the sum components 674 is advantageous because it effectivelycancels any noise sources which are common to both the tone 156 andsideband 154. In addition, processing a single sideband 154U, ratherthan both sidebands 154U and 154L together, prevents any potentialdestructive interference between the sidebands, as may be caused by thefrequency dispersion effects discussed previously.

FIG. 7 is a block diagram of another embodiment 790 of signal extractor190 based on multiplying a tone with a sideband. This extractor 790includes two bandpass filters 710 and 712, a multiplier 720 and a lowpass filter 730. The two bandpass filters 710, 712 are each coupled toreceive the incoming electrical signal 150 and are coupled on theiroutputs to multiplier 720. The multiplier is coupled to low pass filter730.

Bandpass filter 710 selects a tone 156 and bandpass filter 712 selectsone of the sidebands 154. In this specific example, the optical carrierand upper sideband 154U are the selected components. Multiplier 720multiplies the tone 156 against the selected sideband 154U, resulting ina signal with a sum component 774 and a difference component 772, as inFIG. 6. Low pass filter 730 selects the difference component 772, thusrecovering the information signal 140.

FIG. 8 is a block diagram of yet another embodiment 890 of signalextractor 190 using separate extraction paths for different sidebands.Example 890 includes two extraction paths 850A and 850B, and a combiner860. Each extraction path 850 receives the incoming electrical signal150 and is coupled on the output side to combiner 860.

Each extraction path 850 processes a different sideband within theelectrical signal 150 to recover information signals 140A and 140B,respectively. As an example, extraction path 850A might process theupper sideband 154U; whereas extraction path 850B processes the lowersideband 154L. Both extraction paths 850 may use the same tone (e.g.,the optical carrier) in their processing, or they may use differenttones. Combiner 860 receives the recovered information signals 140A and140B and constructively combines them to produce a resultant differencecomponent 140C, which contains the original information signal. Thedifference components 140A and 140B typically may be phase shifted withrespect to each other in order to align their phases before they arecombined; the amount of the phase shift may be frequency-dependent. Ifdifference components 140 are located at difference frequencies,combiner 860 may also frequency shift them to a common frequency beforecombining.

In a preferred embodiment, each path 850 is based on the approach ofsignal extractor 690 of FIG. 6, except that each extraction path 850 isdesigned to process a different sideband. Thus, for example, thebandpass filter 610 for extraction path 850A may be tuned to select theoptical carrier and upper sideband 154U; whereas the bandpass filter 610for extraction path 850B might select the optical carrier and lowersideband 154L. Alternately, each extraction path 850 may be based on theapproach of signal extractor 790 of FIG. 7.

FIG. 9 is a block diagram of one embodiment 990 of signal extractor 890in which the extraction paths 850 share components, although thesidebands are still processed separately. In this embodiment, each ofthe extraction paths 850 is based on signal extractor 790. Extractionpath 850A processes the upper sideband 154U; whereas extraction path850B processes the lower sideband 154L. Both extraction paths use theoptical carrier as the tone 526. Hence, they may share a common bandpassfilter 710, which selects the optical carrier. In other words, theextraction paths are overlapping. The tone 526 is then fed to bothmultipliers 720 in each respective extraction path 850.

Combiner 860 includes a phase shifting element 912 and an adder 914.Phase shifting element 912 phase shifts the difference component 140Aproduced by extraction path 850A so that it is in phase with thedifference component 140B produced by extraction path 850B. Adder 914then adds the two in-phase components to produce the resultingdifference component 140C.

In FIGS. 3-9, the optical carrier played the function of the tone 146.FIG. 10 illustrates an example in which a tone 146 is located at afrequency other than the carrier frequency. In particular, FIG. 10 is adiagram of another embodiment 1010 of transmitter 100 using a pilottone. Transmitter 1010 includes an optical source 112 coupled to an MZM314 as in FIG. 3. However, transmitter 1010 also includes a combiner1020 and a pilot tone generator 1030. The pilot tone generator 1030 iscoupled to one input of combiner 1020, the output of which drives MZM314. The other input of combiner 1020 receives information signal 140.

In transmitter 1010, combiner 1020 combines the pilot tone at afrequency f_(P) with the incoming information signal 140 and uses thecombined signal to modulate MZM 314. If MZM 314 is biased at the V_(π)point, the resulting spectrum 1040 will include upper and lowersidebands 1044 of the information signal, upper and lower sidebands 1048of the pilot tone, and no optical carrier. Each sideband 1048 of thepilot tone may be used by signal extractor 190 as a tone 146. In otherwords, the signal extractor may mix one of the pilot tones 1048 with oneof the sidebands 1044 to recover the information signal 140.

All of the signal extractors 190 described above may be adapted for usewith optical signal 1040. For example, referring to FIG. 6, bandpassfilter 610 may be adjusted to select one of the sidebands 1044 and oneof the pilot tones 1048. The square law device 620 would then produce acorresponding difference component 672. Since this difference componentmight not lie exactly at baseband, low pass filter 630 may also need tobe adjusted in order to recover the correct frequency components.Similarly, referring to FIG. 7, extractor 790 may be adapted for usewith signal 1040 by similarly adjusting the frequency bands for filters710, 712, and 730 to select an appropriate sideband 1044, pilot tone1048 and difference component 772, respectively. Similar adjustments maybe made to the systems discussed in FIGS. 8 and 9. Transmitter 1010 andoptical signal 1040 are merely illustrative, other combinations of tonesand sidebands will be apparent.

FIGS. 11 and 12 are block diagrams of systems 1100 and 1200 illustratingfurther aspects of the invention. Example system 100 used a singlereceiver 130 with a single optical local oscillator signal 134, in orderto illustrate the basic principles of heterodyne detection and ofprocessing sidebands separately. Systems 1100 and 1200 use multiplereceivers, each using an optical local oscillator signal of a differentfrequency. As a result of these different frequencies, each receivereffectively is tuned to a specific wavelength band, thus automaticallyproviding some wavelength selectivity. For clarity, in FIGS. 11 and 12,the term “heterodyne receiver” is used to describe receivers based onheterodyne detection, such as receiver 130 in FIG. 1 and its variantsdescribed in FIGS. 2-10.

In FIG. 11, system 1100 includes a transmitter subsystem 1102 coupled toa receiver subsystem 1104 via an optical fiber 120. Briefly stated, thetransmitter subsystem 1102 encodes information to be transmitted onto anoptical signal. For reasons which will become apparent below, thisoptical signal is referred to as a “composite optical signal.” Thecomposite optical signal is transmitted across the fiber 120 andreceived by the receiver subsystem 1104. The receiver subsystem 1104recovers the original information from the composite optical signal.

In more detail, the transmitter subsystem includes transmitters1110A-1110N which are optically coupled to an optical combiner 1112.Transmitter 110 of FIG. 1 and its variants are suitable for use as atransmitter 1110. Each transmitter 1110 encodes information to betransmitted onto an optical signal which includes sideband(s) ofinformation, as discussed previously in the context of FIG. 1, et seq.Each transmitter 1110 uses a different optical carrier frequencyλ₁-λ_(N) so as to spectrally separate the relevant sidebands of thevarious optical signals. Combiner 1112 optically combines the opticalsignals to produce the composite optical signal. Examples of combiners1112 include 1:N power combiners (i.e., not wavelength selective) andWDM multiplexers. FIGS. 14A and 14B show spectra for two examplecomposite optical signals. Referring first to FIG. 14A, transmitter1110A produces double sideband signal 1410A. This signal includesoptical carrier 1411A at wavelength λ₁ and an upper and lower sideband1412A(U) and 1412A(L), respectively. Similarly, transmitters 1110B-1110Nproduce signals 1410B-1410N. For clarity, each of the sidebands1412A-1412N will be referred to as subbands of the composite opticalsignal. The composite optical signal in FIG. 14B has a similar structureto that in FIG. 14A, except that the constituent optical signals 1420are single sideband signals.

Thus, the composite optical signal includes at least two subbands ofinformation, at least one from each of at least two transmitters 1110.The composite signal also includes at least one tone for use in thesubsequent heterodyne recovery although it typically will contain more.As an example, if the optical carrier corresponding to each subband isused as the tone, then each transmitter will generate one tone and thecomposite optical signal will include a total of N tones. Eachtransmitter 1110 preferably generates the tone for the correspondingsubbands.

On the receive side, the receiver subsystem 1104 includes an opticalsplitter 1132 coupled to heterodyne receivers 1130A-1130N. Ignoreelement 1133 for now. Again, receiver 130 of FIG. 1 and its variants aresuitable for use as a receiver 1130. The optical splitter 1132 splitsthe composite optical signal into N optical signals, from which theencoded information is recovered. Each optical signal includes at leastone subband and one tone, and each heterodyne receiver 1130 recovers theinformation from the subband using the heterodyne techniques describedpreviously. More specifically, each heterodyne receiver 1130 uses anoptical local oscillator at the appropriate frequency to select, ifnecessary, and process the appropriate subband and tone which itreceives. In a preferred embodiment, there is a one-to-onecorrespondence between transmitters 1110 and receivers 1130. The opticallocal oscillator for receiver 1130A is selected to recover the subbandproduced by transmitter 1110A, which is located at an optical carrierfrequency of λ₁. A similar relationship exists for the othertransmitters 1110 and receivers 1130.

System 1100 implements an unconventional type of wavelength divisionmultiplexing (WDM). Each of the optical signals generated bytransmitters 1110 uses a different wavelength λ₁-λ_(N). These differentwavelength signals are combined and then transmitted over a single fiber120. At the receiver subsystem 1104, they are then separated bywavelength and separately processed. However, as a result of theinherent spectral selectivity and increased sensitivity of heterodynedetection, system 1100 is different from conventional WDM systems inmany respects. For example, in a conventional WDM system, the wavelengthseparation is implemented entirely by optical splitter 1132, which wouldbe a WDM demultiplexer. In system 1100, however, the heterodynereceivers 1130 are also wavelength selective. Thus, the opticalcrosstalk suppression requirements of the optical splitter 1132 can beless stringent than those required for conventional WDM systems.

For example, in certain applications, a standard 1:N power splitter isappropriate for optical splitter 1132. Note that a conventional 1:Npower splitter simply splits an incoming signal into N outgoing signals,each with 1/N the power of the original signal. Furthermore, unlike aWDM demultiplexer, a power splitter is not wavelength selective and,therefore, also is not selective between optical signals located atdifferent wavelengths. Thus, for example, a power splitter does notsuppress crosstalk between signals at different wavelengths. As a resultof the large power loss, the lack of wavelength selectivity and thecorresponding lack of crosstalk suppression, power splitters generallyare not preferred for conventional WDM systems. In system 1100, however,the use of heterodyne detection overcomes both of these limitations. Theincreased sensitivity of heterodyne receivers compensates for the largepower loss. The use of an optical local oscillator (and subsequentelectrical filtering) to select the subband and tone of interestcompensate for the lack of wavelength selectivity and crosstalksuppression. In fact, heterodyne receivers can be more wavelengthsensitive than current WDM demultiplexers, thus allowing the opticalcarriers used by transmitters 1110 to be more closely spaced than inconventional WDM systems. As an intermediate solution, optical splitter1132 may have some wavelength selectivity. For example, it may be aconventional 1:N power splitter followed by broad wavelength filters, sothat the optical signals entering each heterodyne receiver 1130 aresomewhat attenuated in the unwanted wavelength bands. Referring to FIG.11, a wavelength filter tuned to wavelength λ₁ may be located at 1133 inorder to filter the signal received by receiver 1130A. This increasesthe wavelength selectivity and also increases the optical signal tonoise ratio since out of band noise is reduced. Alternately, opticalsplitter 1132 may be a WDM demultiplexer which has a spectral responsetoo wide for use in conventional WDM systems but which offers someimprovement over a spectrally flat power splitter.

In FIG. 12, system 1200 is designed to transmit a composite opticalsignal of 40 billion bits per second (Gbps) of digital data across asingle fiber. System 1200 includes a transmitter subsystem 1202 coupledto a receiver subsystem 1204 via an optical fiber 120. In system 1100 ofFIG. 11, each transmitter 1110 received an electrical information signaland generated an optical signal with sidebands. The optical signals werethen optically combined to produce the composite optical signal. Insystem 1200, the information signals and tones are electrically combinedto produce an electrical high-speed channel, which is then converted tooptical form to produce the composite optical signal. Approaches whichuse a mix of electrical and optical combining will be apparent. FIGS.13A-13C illustrate the frequency spectra of various signals in system1200. For clarity, only the relevant portions of these spectra aredepicted in the figures.

In more detail, transmitter subsystem 1202 includes four electricaltransmitters 1208A-1208D which are electrically coupled to an FDMmultiplexer 1209, which in turn is coupled to transmitter 1210. Eachelectrical transmitter 1208 includes the same construction as element245 in FIG. 6B of co-pending U.S. patent application Ser. No.09/405,307, “Optical Communications Networks Utilizing FrequencyDivision Multiplexing,” by Michael W. Rowan, et al., filed Sep. 24, 1999(hereafter, the “FDM Application”). In brief, electrical transmitter1208 includes a QAM modulator (included in element 640 of FIG. 6B)coupled to an FDM multiplexer (elements 642 and 644 in FIG. 6B). Eachelectrical transmitter 1208 receives 64 incoming electrical low-speedchannels 1222, each of which has a data rate of 155 Mbps in thisspecific embodiment. The QAM modulator applies a QAM modulation to eachincoming low-speed channel. The FDM multiplexer combines theQAM-modulated low-speed channels using FDM techniques to form anelectrical channel 1224A-1224D which has a data rate of 10 Gbps and awidth of approximately 5.5 GHz. The frequency spectra of signals 1224Aand 1224D are shown in FIG. 13A. See also FIG. 10D, et seq. in the FDMApplication.

The FDM multiplexer 1209 combines the four 10 Gbps channels 1224 into asingle electrical signal, which for convenience will be referred to asthe electrical high-speed channel 1226. It does this using conventionalFDM techniques, frequency shifting some or all of the 10 Gbps channels1224 to higher carrier frequencies. For example, referring again to FIG.13A, channel 1224A is not frequency shifted, as shown by spectra 1234A,but channel 1224D is frequency shifted up to the 25 GHz range, as shownby spectra 1234D. The embodiment shown in FIG. 12 uses a frequency mixer1228D to frequency shift channel 1224D and also uses mixers 1228B and1228C to frequency shift channels 1224B and 1224C, respectively. Nofrequency mixer is used for channel 1224A since it is not frequencyshifted. Alternate embodiments may frequency shift some, none or all ofthe channels 1224 and devices other than frequency mixers may be used toachieve the frequency shifting. Tones 1237 are added after thisfrequency shifting. In the example of FIG. 13A, each tone 1237 islocated at a slightly lower frequency than its corresponding channel1224. In other embodiments, the tones may be located at otherfrequencies, including for example at frequencies higher than those ofthe corresponding channel. In other embodiments, the tones 1237 may alsobe added at different times during the signal processing and/ordifferent channels may share a common tone. In addition, the electricaltransmitters 1208 may include frequency shifters to move the spectrallocation of channels 1224, for example if they would otherwise overlapwith the tones 1237. In this embodiment, the electrical high-speedchannel 1226 has a total data rate of 40 Gbps and a spectral width ofapproximately 25 GHz, as shown in FIG. 13A. In the embodiment shown inFIG. 12, the FDM multiplexer 1209 also includes filters 1235, whichfilter out unwanted frequency components.

Transmitter 1210 is an E/O converter, which in this embodiment includesa laser 1212 and a Mach-Zender modulator 1214. The laser 1212 generatesan optical carrier at a frequency f_(c) and the MZM 1214 modulates theoptical carrier with the 40 Gbps electrical high-speed channel 1226. Asdescribed previously, the MZM may be operated at a number of differentbias points. In this embodiment, it is biased at a point at or close tothe V_(π) points 404 of FIG. 4. In some applications, it is preferableto bias the MZM at the V_(π) point. For example, if separate pilot tonesare used, reducing or eliminating the optical carrier will save power.In theory, biasing at the V_(π) point should eliminate the opticalcarrier but practical constraints usually result in a reduced butnon-zero optical carrier. The result is a composite optical signal 1242with double sideband modulation and a reduced optical carrier, as shownin FIG. 13B. Note that the composite optical signal 1242 has two opticalsidebands 1243U and 1243L, each including four separate subbands1224A-1224D. As mentioned previously, although this example is based ona double sideband optical signal 1242, single sideband signals may alsobe used. For example, in one embodiment, the lower sideband 1243L ofcomposite signal 1242 is eliminated, for example by optical filtering.The resulting composite signal would occupy half the spectral bandwidth.

On the receive side, the receiver subsystem 1204 includes an opticalsplitter 1232 which is optically coupled to four heterodyne receivers1230A-1230D, each of which is coupled to an electrical receiver 1238.The splitter 1232 splits the received composite signal 1242 into fouroptical signals 1252A-1252D, one for each heterodyne receiver 1230.Accordingly, each optical signal includes a primary subband 1224 ofinterest plus corresponding tone 1237. In this embodiment, the opticalsplitter 1232 is a power splitter with wavelength filters, as describedpreviously. In an alternate embodiment, the optical splitter 1232includes separate splitters, each of which splits off one of the opticalsignals 1252 from the composite signal 1242.

FIG. 13C shows the spectrum for signal 1252A, as an example. The primarysubband 1271A(U) and tone 1272A(U) for optical signal 1252A is locatedin the spectral region located 0-5.5 GHz above the optical carrier. Theprimary subband 1271A(U) and tone 1272A(U) in FIG. 13C correspond tosubband 1224A(U) and tone 1237A(U) in FIG. 13B. The other subbands(subbands 1271A(L) and 1271B(U) are shown in FIG. 13C) are attenuated bythe wavelength filters, which have spectral response shown by the dashedline 1273A. FIG. 13C also shows the frequency spectrum for opticalsignal 1252D with primary subband 1271D(U) and tone 1272D(U), whichcorrespond to subband 1224D(U) and tone 1237D(U) in FIG. 13B. Again, thedashed line 1273D shows the spectral response of the correspondingwavelength filter.

In the embodiment shown, the heterodyne receivers 1230 recover theoriginal electrical signals 1254 from the incoming optical signals 1252.Continuing the trace of signals through receiver 1230A, receiver 1230Auses an optical local oscillator which is located at a frequency whichis 11.5 GHz removed from the optical carrier frequency.

Spectrum 1253A of FIG. 13D shows the relevant portions of the signalafter the optical local oscillator has been combined with the incomingoptical signal 1252A and then detected by the square law detector. Thespectrum 1253A includes several frequency components 1261-1265.Frequency components 1261, 1262A and 1263A are the frequency-offsetversions of the optical carrier, subbands 1271A and tones 1272A,respectively.

Using a square law detector, frequency components 1264 and 1265 resultfrom the direct detection of the received signal. For convenience, thesecomponents 1264 and 1265 shall be referred to as direct detectioncomponents. In particular, frequency component 1264 generally includesthe direct detection cross-products of the subband of interest and toneswhich are located close in frequency, for example the cross-product ofsubband 1271A(U) with tone 1272A(U). Frequency component 1264 may alsoinclude the direct detection cross-products of other subbands and tonesif they have not been significantly filtered, for example thecross-product of subband 1271B(U) with tone 1272B(U). Frequencycomponent 1265 generally includes the direct detection square-productsof the subband of interest, for example the square product of 1271A(U)in this example. It may also include direct detection cross-products ofsubbands with each other, for example the cross-product of subband1271A(U) with 1271A(L).

Note that frequency components 1264 and 1265 typically represent themost significant unwanted frequency components, but not the onlyunwanted frequency components. For example, cross-products of tones andcarriers are not shown in FIG. 13D. Neither are all of the possiblecross-products which theoretically could be generated from the squarelaw detection. Although not shown in FIG. 13D, all of these frequencycomponents are accounted for in the overall design, typically either byensuring that they fall outside the frequency band of interest or bysufficiently attenuating them (or the frequency components which giverise to them) so that they are negligible.

Spectrum 1253A illustrates an embodiment in which the local oscillatoris selected so that the direct detection components 1264 and 1265 do notoverlap with the primary subband 1262A(U). In the example given here,the frequency offset is 11.5 GHz, but any suitable offset may be chosen.In an alternate embodiment, the direct detection components 1264 and/or1265 may overlap with the primary subband 1262A(U) so long as theresulting crosstalk is tolerable.

The subband 1262A(U) is frequency filtered and frequency down-shifted toapproximately the 0-5.5 GHz spectral location by using component1263A(U) as the tone in the signal extractor, yielding the electricalsignal 1254A, as shown in FIG. 13D. The frequency filtering also reducesthe noise which results from the local oscillator beating with opticalnoise in the signal.

Optical signal 1252D is similarly processed, as shown in FIG. 13E. Morespecifically, the local oscillator for heterodyne receiver 1230D isselected to be 11.5 GHz offset from tone 1272D(U). Spectrum 1253D ofFIG. 13E shows the signal after this optical local oscillator has beencombined with the incoming optical signal 1252D and then detected by thesquare law detector. Frequency components 1263D(U), 1262D(U) and1262C(U) are the frequency-offset versions of the tone 1272D(U), theprimary subband 1271D(U) and other subband 1271C(U), respectively.Frequency components 1264 and 1265 are direct detection components. Thesubband 1262D(U) is frequency filtered and frequency down-shifted to the0-5.5 GHz spectral location, yielding the electrical signal 1254D, asshown in FIG. 13E.

Note that since receiver subsystem 1204 splits the composite signal 1242into four signals, each of which is processed by a different heterodynereceiver 1230, each heterodyne receiver can have a narrower spectralresponse than if the entire composite signal were processed by a singlereceiver. In this case, each heterodyne receiver 1230 recovers a signalof approximately 5 GHz spectral width and requires a similar spectraloperating range; whereas the composite signal has a sideband width ofapproximately 25 GHz.

Electrical receiver 1238 reverses the functionality of electricaltransmitter 1208, separating the incoming 5.5 GHz electrical signal 1254into its 64 constituent 150 Mbps low-speed channels 1256. Accordingly,each receiver 1238 includes the same construction as element 240 in FIG.6A of the FDM Application. An FDM demultiplexer (elements 624 and 622 inFIG. 6A) frequency demultiplexes the 5.5 GHz electrical signal 1254 into64 separate electrical channels, each of which is then QAM demodulatedby a QAM demodulator (included in element 620 in FIG. 6A).

System 1200, like the other systems described, is an example. Theinvention is not limited to the specific numbers of transmitters and/orreceivers, frequency ranges, data rates, etc. Other variations will beapparent. For example, a 40 Gbps transmitter subsystem 1202 operating ata first wavelength λ₁ could be used as the transmitter 1100A in system1100, a second transmitter subsystem 1202 operating at wavelength λ₂ astransmitter 1100B, and so on, with corresponding changes on the receiveside. In this way, systems 1100 and 1200 can be combined to yield aneven higher data rate system.

As another example, FIG. 13B illustrates one example composite signal1242 in which each subband 1224 has a corresponding tone 1237 and thetones 1237 are located between the optical carrier and the correspondingsubband 1224. FIGS. 15A-15D illustrate other types of composite opticalsignals. In FIG. 15A, the composite signal 1502 is similar to compositesignal 1242 of FIG. 13B with one difference. The tone 1507A for theinnermost subbands 1504A is located at the same frequency as the opticalcarrier rather than at a separate frequency, as is the case with tones1237A in FIG. 13B. In FIG. 15B, the composite signal 1512 is similar tocomposite signal 1242 except that there is a wide spectral separation1503 between the subbands 1514 and optical carrier 1500. In FIG. 15C,each subband 1534 is located between the optical carrier 1500 and thecorresponding tone 1537, instead of vice versa as in FIG. 13B. As afinal example, in FIG. 15D, the tones 1547 are shared by subbands 1544.For example, tone 1547A(U) corresponds to both subband 1544A(U) andsubband 1544B(U). Other variations will be apparent.

FIG. 16 is a block diagram of a system 1600 illustrating yet anotheraspect of the invention. System 1600 includes a transmitter subsystem1602 coupled to a receiver subsystem 1604 via an optical fiber 120. Thetransmitter subsystem 1602 includes two optical transmitters 1610A and1610B, an optical combiner 1614, and an optical filter 1615. Each of theoptical transmitters 1610 is coupled to the optical combiner 1614, whichin turn is coupled to the optical filter 1615.

System 1600 operates as follows. Each optical transmitter 1610 producesan optical signal 1660A or 1660B, respectively, which eventually istransmitted down the fiber as an optical single sideband signal. Eachoptical signal 1660 includes one or more subband(s) and tone(s) foreventual heterodyne detection. In this particular example, opticalsignal 1660A is a double-sideband signal having an upper opticalsideband 1668A(U), a lower optical sideband 1668A(L), and a suppressedcarrier 1669A. In an alternate embodiment, the carrier 1669A may not besuppressed in the optical transmitter 1610, but suppressed later, forexample by optical filter 1615. Upper sideband 1668A(U) includessubbands 1662A(U) and 1666A(U), and tone 1664A(U). Lower sideband1668A(L) includes the mirror image: subbands 1662A(L) and 1666A(L), andtone 1664A(L). Note that in this example, the subbands 1662A and 1666Aare not upper and lower sidebands resulting from an electrical doublesideband modulation in which signal 1664A is an electrical carrier.Rather, each subband 1662A and 1666A carries different information andsignal 1664A is a tone.

Optical signal 1660B is similarly structured, containing two opticalsidebands 1668B and a suppressed carrier 1669B. Each optical sideband1668B includes two subbands 1662B and 1666B, and a tone 1664B. Thesubbands 1662B and 1668B are different from the subbands 1662A and1666A; so in this example, there are a total of four subbands carryingdifferent information. Optical signals 1660A and 1660B are alsodifferent in that they are orthogonally polarized. In one embodiment,they have crossed linear polarizations. In FIG. 16, the orthogonalpolarizations are indicated by the orientation of the spectra. Forexample, spectra 1660A is oriented in the plane of the paper, indicatingone polarization; while spectra 1660B is oriented coming out of thepaper, indicating an orthogonal polarization. In addition, the twooptical signals 1660 use optical carriers 1669 of different wavelengths.In an alternate embodiment, the optical signals 1660 are orthogonallypolarized but not using crossed linear polarizations. For example, onesignal 1660A may be right circularly polarized; whereas the other signal1660B is left circularly polarized. In another embodiment, the twooptical signals have different polarizations but may not be completelyorthogonally polarized to each other.

The two optical signals 1660 are combined using combiner 1614. Thecombiner 1614 preferably is a polarized beam combiner, so that opticalsignals 1660 are minimally attenuated. In this example, the opticalcarriers 1669 are selected so that in the combined signal 1680, theupper optical sideband 1668A(U) of one signal is adjacent to the lowersideband 1668B(L) of the other signal.

Optical filter 1615 filters out the redundant sidebands: lower sideband1668A(L) and upper sideband 1668B(U) in this case. Filter 1615 may alsosubstantially attenuate the carriers 1669, particularly if, for example,the optical transmitters 1610 do not significantly suppress the carriers1669. In this example, the optical filter 1615 is shown on the transmitside 1602, located after the optical combiner 1614. However, filteringtypically can be implemented at a number of different locations and/ordistributed between different locations. For example, an optical filtermay also be placed on the receiver side, between fiber 120 and opticalsplitter 1632. One advantage of this placement is that this opticalfilter can also filter out noise generated during transmission, such asamplified spontaneous emission. In WDM applications, filters can also beused to suppress unwanted channels. As a final example, optical filterscan be placed between the optical transmitters 1610 and optical combiner1614 to filter out the unwanted sidebands and/or suppress the opticalcarriers.

In one embodiment, optical filter 1615 is a simple optical bandpassfilter. In another embodiment, the optical filter 1615 is implemented asa comb filter, or a series of comb filters. Comb filters have periodicalternating pass and stop bands which repeat on a regular basis. Forexample, a comb filter might have alternating pass and stop bands, withthe spectral response repeating with a periodicity of 100 GHz as shownin FIG. 26. Put in another way, the comb filter in FIG. 26 has passbands spaced on 100 GHz centers. In one embodiment, the comb filter isimplemented as an interleaver, which can also be used to combine sets ofwavelengths in WDM applications. One advantage of using comb filters,including interleavers, is that tunable optical carriers 1669 can beaccommodated. If the wavelength of the optical carriers 1669 is changed,the transmitter subsystem 1602 is still functional so long as thedesired subbands fall in one of the pass bands of the comb filter (orseries of comb filters).

For example, most WDM standards specify a grid of wavelengths in whichthe wavelengths are regularly spaced. By using a comb filter matched tothis spacing, any of the wavelengths in the grid can be accommodated.This is because a comb filter has multiple pass bands which areperiodically spaced. For example, the optical transmitters 1610 may bebased on optical sources in which the wavelength is tunable to differentwavelengths in the grid (e.g., a tunable laser). In contrast, an opticalbandpass filter typically only has a single pass band. If the wavelengthof the optical carriers 1669 is changed, this typically will require adifferent bandpass filter matched to the new wavelengths (or at leasttuning of the location of the pass band).

To use a numerical example, assume that the WDM standard specifies aspacing of 100 GHz between different wavelength channels. By using acomb filter with the same periodicity, such as the one in FIG. 26, anyof the wavelength channels can be accommodated. In a differentembodiment, two comb filters are used, each with a periodicity of 200GHz. One comb filter handles the even wavelength channels and the othercomb filter handles the odd wavelength channels.

The resulting composite optical signal 1690 includes the upper sideband1668A(U) from optical signal 1660A and the orthogonally polarized lowersideband 1668B(L) from optical signal 1660B. Each of the four subbandsof composite optical signal 1690 carries different information, forexample a different 10 Gbps data stream in one embodiment. Note thatcomposite optical signal 1690 is a single sideband signal in that onlyone optical sideband of each subband is transmitted. The other opticalsideband was removed by filter 1615. System 1600 is merely one exampleof an approach capable of generating optical single sideband signals.For example, see FIGS. 3-5 of co-pending U.S. patent application Ser.No. 09/747,261, “Fiber Optic Communications using Optical SingleSideband Transmission and Direct Detection,” by Ting K. Yee and Peter H.Chang, filed Dec. 20, 2000.

On the receive side, the receiver subsystem 1604 is similar to receiversubsystems 1104 and 1204. This particular receiver subsystem 1604includes an optical splitter 1632 coupled to four heterodyne receivers1630A-D. Each receiver 1630 recovers one of the four subbands 1662A,1662B, 1666A or 1666B using heterodyne techniques, for example asdescribed previously. Subbands 1662A and 1666A each use tone 1664A inthe heterodyne detection. In other words, the tone 1664A is shared bytwo subbands. Similarly, subbands 1662B and 1666B share tone 1664B. Thesplitter 1632 splits the received composite optical signal 1690 intofour optical signals 1692A-D, one for each heterodyne receiver 1630.Each optical signal 1692 includes the relevant subband plus tone. Asbefore, the polarization controller within the receivers 1630 matchesthe polarization of the local oscillator to the polarization of thetone. When multiple subbands share the same tone, placing the tone inthe middle of the subbands is preferred. Thus, the frequency separationbetween the tone and the furthest subband is minimized, therebyminimizing the attenuation of the detected electrical signal due topolarization mode dispersion.

In the embodiment shown in FIG. 16, the optical splitter 1632 includes apolarizing beam splitter module 1633 coupled to two optical splitters1634A-B, each of which is coupled to an optical filter 1635A-D. Thepolarizing beam splitter module 1633 directs signal 1668A(U) to splitter1634A and orthogonally polarized signal 1668B(L) to splitter 1634B. Eachoptical splitter 1634 further divides the incoming signal and thecorresponding filters 1635 filter the appropriate subband and tone. Forexample, splitter 1634A splits the signal 1668A(U) into two identicalsignals, each of which is directed to one of the optical filters1635A-B. Filter 1635A attenuates unwanted subband 1666A(U) and passessubband 1662A(U) and tone 1664A; while filter 1635B attenuates unwantedsubband 1662A(U) and passes subband 1666A(U) and tone 1664A.

FIG. 17 is a block diagram of one embodiment of optical transmitter1610. This embodiment is similar to the transmitter subsystem 1202 ofFIG. 12, with the following exceptions. First, in this particularexample, there are only two electrical transmitters 1708 rather than thefour shown in FIG. 12. In addition, the electrical transmitters 1708 mayfrequency division multiplex incoming electrical low-speed channels 1722into the electrical channels 1724A-B. However, they may instead combinelow-speed channels using other techniques, such as time divisionmultiplexing, or they may not do any combining at all. For example, theincoming signal 1722 may simply be passed through to form channel 1724,in which case there is no need for electrical transmitter 1708. Second,the transmitter subsystem 1610 further includes a polarizationcontroller 1715.

Briefly, the optical transmitter 1610 operates as follows. Eachelectrical transmitter 1708 produces an electrical channel 1724. The twoelectrical channels 1724A and 1724B correspond to the subbands 1662 and1666 The FDM multiplexer 1709 combines the channels 1724 into a singleelectrical signal 1726 using conventional FDM techniques. A tone, whichcorresponds to tone 1664, is also added. The FDM multiplexer 1709 alsoincludes filters, which filter out unwanted frequency components. Theelectrical signal 1726 entering the E/O converter 1710 includes twosubbands and a tone.

In this embodiment, the E/O converter 1710 includes a laser 1712, aMach-Zender modulator 1714, and a polarization controller 1715. Thelaser 1712 generates an optical carrier and the MZM 1714 modulates theoptical carrier with the incoming electrical signal 1726. As describedpreviously, the MZM may be operated at a number of different biaspoints. In this embodiment, it is biased at a point close to the V_(π)points 404 of FIG. 4. Since the electrical signal 1726 has its own tone,reducing the optical carrier saves power. The result from MZM 1714 is adouble sideband signal.

The polarization controller 1715 controls the polarization of thissignal, yielding the optical signal 1660 with upper and lower opticalsidebands 1668 and a reduced optical carrier 1669. In one approach, thepolarization controller 1715 is a polarization rotator. In anotherembodiment, the output of the MZM 1714 is coupled topolarization-preserving fiber, which is physically rotated or twisted toachieve the desired polarization rotation. In a preferred embodiment,only one of the two optical transmitters 1610 requires a polarizationcontroller 1715, in order to manipulate the polarization of one signal1660 to be orthogonally polarized to the other signal 1660.

This approach has many benefits. For example, since optical signals1668A(U) and 1668B(L) are orthogonally polarized, their interaction issignificantly reduced. This, in turn, reduces unwanted nonlinear effectsbetween differently polarized components, such as those due to four-wavemixing and cross-phase modulation. In addition, composite optical signal1690 is generated by producing two optical signals 1660A and 1660B withseparate carriers 1669A and 1669B but with sidebands 1668A(U) and1668B(L) which are close to each other. The desired sidebands areselected by filtering. This approach allows the sidebands and theirsubbands to be more densely spaced in comparison to wavelength divisionmultiplexing approaches.

As a final example, the subbands within each sideband are assembledusing frequency division multiplexing in the electrical domain.Assembling subbands via frequency division multiplexing also results inmany benefits, such as dense spacing of the subbands, efficientbandwidth utilization (both as a result of dense spacing and efficientmodulation techniques), less susceptibility to frequency-dependenteffects, non-linear fiber effects, and polarization mode dispersion(since each subband is concentrated over a narrow frequency band), andthe ability to easily handle channels of different data rates andprotocols.

Referring again to FIG. 16, the two optical carriers 1669 are selectedso that the upper sideband 1668A(U) and lower sideband 1668B(L) areclose enough to each other that they can be selected by filter 1615. Toachieve this, the two optical sources 1712 in the two opticaltransmitters 1610 preferably are wavelength-locked to each other. It iseven more desirable for each optical source 1712 to be wavelength-lockedto a specific wavelength, in which event the difference between the twosources 1712 would also remain constant.

FIG. 18A is a block diagram of a wavelength locking device 1800 forwavelength locking optical source 1712A to a specific wavelength. Forconvenience, only the relevant portions of transmitter subsystem 1602are reproduced in FIG. 18A. The following additional components are alsoshown in FIG. 18A: a sinusoidal generator 1840 at frequency f1, opticaltaps 1816 and 1817, photodetectors 1820 and 1818, synchronous detectors1824 and 1822, and comparison circuitry 1834.

These components are coupled as follows. The sinusoidal generator 1840is coupled to the electrical input of MZM 1714A. It is also coupled toboth synchronous detectors 1824 and 1822. One optical tap 1817 islocated before the optical filter 1615, and the other optical tap 1816is located after the optical filter 1615. Tap 1817 is coupled tophotodetector 1818 to synchronous detector 1822 to comparison circuitry1834. Similarly, tap 1816 is coupled to photodetector 1820 tosynchronous detector 1824 to comparison circuitry 1834. The output ofcomparison circuitry 1834 is coupled to the optical source 1712A.

Wavelength-locking of optical source 1712A occurs as follows. Thesinusoidal generator 1840 produces a reference signal at a frequency f1,which preferably is low, for example in the kHz range. The opticalsignal 1660A is modulated at this low frequency f1. In FIG. 18A, themodulation is achieved by adding the reference signal at frequency f1 tothe electrical signal driving the MZM 1714A. In an alternativeembodiment, the laser 1712A is directly modulated by the referencesignal or by amplitude modulating the laser 1712A. Amplitude modulationgenerally results indirectly in frequency modulation. Direct lasermodulation generally requires that the heterodyne detector canaccommodate the associated frequency excursions of the laser. Whateverthe method, the optical signal 1680 includes a small component at afrequency f1 offset from the optical carrier 1669A.

Optical tap 1817 taps a small portion of the optical signal 1680, priorto propagating through the filter 1815. This is detected byphotodetector 1818, which results in mixing of the various frequencycomponents in the tapped signal. Synchronous detector 1822 receivesthese various frequency components and also receives a reference signalat frequency f1. The synchronous detector 1822 locks in to the secondharmonic component at 2 f1 (other harmonics, the fundamental orsubharmonics can also be used) and outputs a signal proportional to thestrength of this frequency component. Examples of synchronous detector1822 include lock-in amplifiers and digital circuitry for implementingthe same functionality. The output signal also indicates the strength ofthe optical carrier 1669A before filtering, since the frequency f1 isnegligible with respect to that of the optical carrier. In a similarfashion, the optical tap 1816, photodetector 1820 and synchronousdetector 1824 generate an output signal which indicates the strength ofthe optical carrier 1669A after filtering by optical filter 1615.

Comparison circuitry 1834 receives the signals from the two synchronousdetectors 1822 and 1824 and compares them. In this particular example,the comparison circuitry take the ratio of the two signals. The ratioindicates the attenuation experienced by the optical carrier 1669A as itpropagates through optical filter 1615, which in turn is a function ofthe wavelength of the optical carrier 1669A. Therefore, based on thisratio, the comparison circuitry 1834 generates an error signal which isused to adjust the wavelength of optical source 1712A.

As a specific example, FIG. 18B shows the transfer function of opticalfilter 1615 as a function of wavelength. Assume that optical carrier1669A nominally is located at the wavelength 1870 on the lower edge ofthe filter transfer function and, at this wavelength, the filter 1615has a 6 dB optical attenuation. In this implementation, this correspondsto a 12 dB attenuation of the relevant electrical signal. If the actualratio is 12.1 dB of electrical attenuation, then the optical carrier1669A must be at a wavelength lower than nominal and comparisoncircuitry 1834 generates an error signal to increase the wavelength.Similarly, if the actual ratio is only 11.9 dB of attenuation, then theerror signal decreases the wavelength.

The same approach is used to wavelength-lock the optical carrier 1669Bgenerated by optical source 1712B. In FIG. 18B, the nominal wavelengthof carrier 1669B is also located at 6 dB of attenuation, but at theupper edge of the filter transfer function. Thus, too much attenuationmeans the wavelength is too high, and too little attenuation means thewavelength is too low.

FIG. 18C is a graph illustrating an implementation of optical filter1615 based on Bragg grating filters. A single Bragg grating filtergenerally passes light except at one narrow band (i.e., around thewavelength at which the reflected light constructively interferes). Inother words, the Bragg grating filter acts as a notch filter intransmission. The optical filter 1615 in FIG. 18C includes two Bragggrating filters. The first filter has a notch at location 1880 and thesecond filter has a notch at location 1882. Thus, the overall spectralcharacteristic includes the two notches at 1880 and 1882, as well as apass band 1884 between the two notches. The notches and pass band areselected so that the pass band 1884 passes the two sidelobes 1668A(U)and 1668B(L), the two optical carriers 1669A and 1669B fall on the edgesof the pass band, and the two notches block the redundant sidelobes1668A(L) and 1668B(U). In a preferred embodiment, the Bragg gratingfilters are implemented as fiber Bragg grating filters. Otherimplementations, for example interleavers and thin film filters, will beapparent.

As usual, the wavelength locking device 1800 in FIG. 18A is merely anexample. Other approaches to wavelength-locking may also be used,including those discussed in co-pending U.S. patent application Ser. No.09/746,261, “Wavelength-Locking of Optical Sources,” by Shin-ShengTarng, et al., filed Dec. 20, 2000.

FIGS. 19 and 20A illustrate another example embodiment 1900 of opticaltransmitter 1610. FIG. 19 is a block diagram of the optical transmitter1900 and FIG. 20A shows spectra at various points in the opticaltransmitter 1900. This optical transmitter 1900 is designed to receiveeight OC-48 signals 1922A-H and combine them into a single electricalsignal 1926 with eight subbands 1962A-H (one for each OC-48 signal) anda single shared tone 1964.

The optical transmitter 1900 includes the following components. Thereare eight channels, one for each incoming OC-48 signal 1922, with eachchannel including an OC-48 transceiver 1902, error correction encoder1904, and modulator 1906 coupled in series. The eight channels arecoupled to four combiners 1908, two channels coupled to each combiner.Each combiner enters a frequency upconverter 1910, all of which arecoupled to a final combiner 1912.

The optical transmitter 1900 operates as follows. Each OC-48 signal 1922is transformed from optical to electrical signal by the transceiver1902. The resulting spectrum 2010 is shown in FIG. 20A. The electricalsignal is encoded by the FEC 1904 using a forward error correction codeand then QPSK modulation is used in modulator 1906 to modulate an RFcarrier. In more detail, QPSK modulator 1906 encodes the incoming dataas I and Q channels, which are then used to modulate an electricalcarrier. In this particular example, alternate channels use the samefrequency carrier. That is channels A, C, E and G use one carrier,resulting in the spectrum 2012; but channels B, D, F and H producespectrum 2014. The combiners 1908 combine the channels, two at a time,producing spectra 2016. These four signals are frequency shifted todifferent frequency locations by the frequency upconverters 1910 andthen combined by combiner 1912 to yield the electrical signal 1926. Thissignal contains eight subbands 1962A-H, generated from the originalspectra 2016. A tone 1964 is also added. This electrical signal is thenfed to the E/O converter 1710, where it is processed the same as in FIG.17. As mentioned previously, this is but one example of an opticaltransmitter. For example, other architectures for frequency divisionmultiplexing the channels together will be apparent.

FIG. 20B shows the spectra of the composite optical signal 1990generated by the overall system. As described above, one opticaltransmitter 1900A produces the signal group 2168A, which contains eightsubbands and a tone. The optical transmitter 1900A also generates areduced optical carrier 2169A, which may be further reduced by opticalfiltering. Similarly, the other optical transmitter 1900B producesoptical carrier 2169B and signal group 2168B, with eight other subbandsand a tone. In this particular example, each subband carries theequivalent of an OC-48 signal and is approximately 2.5 GHz wide so eachsignal group 2168 is a total of approximately 20 GHz wide. There is a 4GHz guard band between the two groups, yielding a total spectral widthof approximately 44 GHz for the composite optical signal. The opticalcarriers 2169 are offset by another 20 GHz, for a total carrier tocarrier width of 84 GHz. Optical filtering suppresses the carriers,resulting in an effective signal width of 44 GHz (i.e., the bandwidth ofthe composite optical signal). The total capacity of the system issixteen OC-48 subbands, or a total of approximately 40 Gbps. On thereceive-side, heterodyne detection is accomplished by placing the localoscillator signal for a subband at the frequency of the correspondingcarrier. The separation between local oscillator and subbands istherefore 20 GHz and is selected so that direct detection signals do notoverlap spectrally with constituent signals on heterodyne detection, asdescribed previously.

FIG. 27 is a block diagram of another embodiment 2700 of opticaltransmitter 1610. This optical transmitter 2700 is designed to receiveeight OC-48 signals 2722A-H and combine them into a single electricalsignal 2726, as does optical transmitter 1900. However, the combining isimplemented differently.

The optical transmitter 2700 includes the following components. Thereare eight channels, one for each incoming OC-48 signal 2722, with eachchannel including an OC-48 transceiver 2702 and error correction encoder2704 coupled in series. The eight channels then share four modulators2706, two channels per modulator. The four modulators 2706 are coupledto an FDM multiplexer 2709, which could be similar in construction tothe combination 1908-1910-1912 shown in FIG. 19.

The optical transmitter 2700 operates as follows. Each OC-48 signal 2722is transformed from optical to electrical signal by the transceiver2702. The electrical signal is encoded by the FEC 2704 using a forwarderror correction code. QPSK modulators 2706 encode the eight incomingdata streams as four I channels and four Q channels, and each I/Q pairis used to modulate an electrical carrier. The result is four QPSKmodulated signals. Note that the OC-48 signals may be asynchronous withrespect to each other. These four subbands plus a tone are combined byFDM multiplexer 2709 to yield the electrical signal 2726. This signalcontains four subbands 2762A-D and a single shared tone 2764. Eachsubband 2762 contains the data from two OC-48 signals, for a data rateof approximately 5 Gbps per subband 2762. This electrical signal is thenfed to the E/O converter 1710, where it is processed the same as in FIG.17. A second similar optical transmitter produces a second, orthogonallypolarized optical signal. The two optical signals are combined togenerate a composite optical signal. The resulting composite opticalsignal contains two orthogonally polarized optical signals, each havingthe structure of four subbands plus tone.

FIG. 21 is a block diagram of another embodiment 2500 of receiversubsystem 1604. This particular receiver subsystem 2500 includes anoptical splitter 2532 coupled to two heterodyne receivers 2530A-B. Eachreceiver 2530 processes one of the two orthogonally polarized signals1668A(U) and 1668B(L), respectively, using heterodyne techniques, forexample as described previously. Subbands 2562A and 2566A use tone 2564Ain the heterodyne detection. Similarly, subbands 2562B and 2566B usetone 2564B. The splitter 2532 splits the received composite opticalsignal 2590 into two optical signals 2592A-B, one for each heterodynereceiver 2530. Each optical signal 2592 includes the relevant twosubbands plus tone. Placing the tone between the two subbands reducesthe frequency separation between tone and subband, thereby minimizingthe attenuation of the detected electrical signal due to polarizationmode dispersion. In this implementation, the optical splitter 2532includes optical splitter coupled to two optical filters 2535A-B. Asbefore, the polarization controller in the receivers 2530 matches thepolarization of the local oscillator to the polarization of the tone.

FIG. 22 is a block diagram of another embodiment 2200 of transmittersubsystem 1602 utilizing a birefringent medium. This embodiment includesa laser 2202, modulator 2206, birefringent medium 2210, and opticalfilter 2215 coupled in series. Light from optical source 2202 passesthrough modulator 2206 to generate optical signal 2250. In thisparticular example, the information signal 2248 entering the MZMcontains sixteen subbands, each based on an OC-48 signal, for a totalbandwidth of approximately 40 GHz. The optical signal 2250 generated bythe MZM 2206 is a double sideband version of this, with a suppressedcarrier. Birefringent medium 2210 has group velocity dispersion. Hence,when optical signal 2250 passes through birefringent medium 2210, eachsubband experiences a different degree of phase retardation between thebirefringent axes. In a preferred embodiment, birefringent medium 2210is a birefringent fiber. Other polarization controlling devices whichintroduce varying polarizations can also be used. The optical signal2260 leaving the birefringent medium still has two optical sidebands.Optical filter 2215 substantially attenuates the redundant sideband andmay further suppress the carrier, resulting in composite optical signal2290. In an alternate embodiment, the optical filter 2215 is locatedbefore the birefringent medium 2210.

FIG. 23 shows composite optical signal 2290 in more detail. The graphshows power as a function of frequency and the figures below the graphillustrate the corresponding polarization states. As shown in the powerspectrum, the optical signal 2290 includes sixteen subbands (orchannels) 2294A-2194P, tone 2296, and suppressed carrier 2297. The loweroptical sideband of optical signal 2290 is substantially attenuated. Asa consequence of the birefringence, the polarization of each channel isslightly different. In this example, channel 1 (i.e., 2794A) is linearlypolarized 2298A and channel 16 (2794P) is similarly linearly polarized2298E. In between, the phase retardation varies continuously, so thatthe polarization gradually transforms from linear vertical 2298A, toright circular 2298B, to linear horizontal 2298C, to left circular2298D, back to linear vertical 2298E. The polarizations generally areelliptical. The varying polarization serves to reduce four-wave mixingand cross-phase modulation between the channels.

As is noted throughout, the systems described herein are merelyexamples. It is not feasible to explicitly describe all possibleembodiments which are based either on the principles illustrated or oncombinations of these principles. For example, systems 1200, 1600, 1900and 2700 discuss at length the use of electrical frequency divisionmultiplexing for combining signals. System 1100, 1900 and 2700 discussesthe use of carriers at different wavelengths for combining signals.Systems 1600, 1900, 2200, 2500 and 2700 discuss the use of differentpolarizations in order to improve performance. Systems 1600, 1900 and2700 discuss at length the use of optical single sideband transmission.System 1600 discusses at length the approach of using separated opticalcarriers to produce sidebands which are close to each other, and thenselecting the desired sidebands via an optical filter. System 2200discusses at length the approach of using a single optical transmitterto produce a signal with multiple subbands of varying polarizations.However, this does not imply that each technique can only be used in thesystems which discuss the technique at length or only in thecombinations which are explicitly illustrated.

For example, FIGS. 24A-25B illustrate other types of composite opticalsignals. FIG. 24 illustrates various signals which include foursubbands. The corresponding tones, if any, have been omitted forclarity. In FIG. 24A, the composite optical signal 2412 includes foursubbands of the same polarization situated near each other. Forcomposite optical signal 2412, the spectral width of the guard band(i.e. gap) between the subbands is generally less than the spectralwidth of the subbands. Note that this signal could be generated by usingthe two optical carrier approach shown in FIG. 16, but without thecross-polarization. In FIG. 24B, the four subbands are situated neareach other, but they are divided into two groups of two subbands eachand the two groups are orthogonally polarized with respect to eachother. In FIG. 24C, the subbands are again divided into two groups oftwo subbands each, but here the groups are widely separated from eachother. Because of the wide separation of the subbands, composite opticalsignal 2416 will experience substantially less cross-phase modulationand four-wave mixing between the more widely separated subbands. FIG.24C shows the subbands as the same polarization, but they could alsohave varying polarizations. As a final example, FIG. 24D shows acomposite optical signal 2428 in which the subbands alternate inpolarization. In other words, the subbands of one polarization areinterleaved with the subbands of the orthogonal polarization.

FIG. 25 illustrates various ways of filtering optical signals based ontwo separate optical carriers. These are illustrated in the context ofoptical signals 1660A and 1660B of FIG. 16. In FIG. 16, the opticalcarriers 1669 for these two signals was selected so that upper sidelobe1668A(U) was adjacent to but at a lower frequency than lower sidelobe1668B(L). In FIG. 25A, these two sidelobes are still adjacent, but lowersidelobe 1668B(L) is now at a lower frequency than upper sidelobe1668A(U). The optical filter 1615, shown by the dashed outline, selectsboth sidelobes. In FIG. 25B, the two upper sidelobes 1668A(J) and1668B(U) are adjacent to each other and selected by the filter 1615.

Although the invention has been described in considerable detail withreference to certain preferred embodiments thereof, other embodimentsare possible. Therefore, the scope of the appended claims should not belimited to the description of the preferred embodiments containedherein.

1. An optical communications system comprising: a transmitter subsystem comprising: at least two optical transmitters, each for generating an optical signal containing a subband of information, each optical signal having a different polarization; and an optical combiner coupled to the optical transmitters for optically combining the optical signals into a composite optical signal.
 2. The optical communications system of claim 1 wherein the optical signals are orthogonally polarized.
 3. The optical communications system of claim 2 wherein: each optical transmitter comprises: an optical source for producing an optical carrier; and an electro-optic modulator coupled to the optical source for modulating the optical carrier with the subband of information; and at least one of the optical transmitters further comprises: a polarization controller for making a polarization of the optical signal orthogonal to a polarization of the other optical signal.
 4. The optical communications system of claim 2 wherein: at least one of the optical transmitter comprises: a wavelength-tunable optical source, whereby a wavelength of the optical signal can be tuned by tuning the wavelength-tunable optical source; and the transmitter subsystem further comprises: a comb filter having periodically spaced pass bands coupled to the optical combiner.
 5. The optical communications system of claim 1 wherein: each optical signal has a lower optical sideband and an upper optical sideband; and each optical transmitter comprises: an optical filter for selecting one optical sideband from the optical signal.
 6. The optical communications system of claim 1 wherein: each optical signal has a lower optical sideband and an upper optical sideband; and the transmitter subsystem further comprises: an optical filter coupled to the optical combiner for selecting one optical sideband from each optical signal.
 7. The optical communications system of claim 6 wherein the optical filter comprises: two Bragg filters coupled in series.
 8. The optical communications system of claim 6 wherein the optical filter comprises: a comb filter having periodically spaced pass bands.
 9. The optical communications system of claim 6 wherein the optical filter selects a lower optical sideband from one optical signal and an upper optical sideband from a different optical signal.
 10. The optical communications system of claim 6 wherein the optical filter attenuates out-of-band wavelengths.
 11. The optical communications system of claim 1 wherein the transmitter subsystem further comprises: a wavelength-locking device coupled to the optical transmitters for locking a frequency separation of the optical signals to a predetermined value.
 12. The optical communications system of claim 11 wherein: each optical signal has a lower optical sideband and an upper optical sideband; and the transmitter subsystem further comprises: an optical filter coupled to the optical combiner for selecting a lower optical sideband from a first optical signal and an upper optical sideband from a second optical signal; a first optical tap coupled between the optical combiner and the optical filter for tapping a portion of the combined optical signals leaving the optical combiner; a second optical tap coupled to the optical filter for tapping a portion of a composite optical signals leaving the optical filter; and the wavelength-locking device is coupled to the first optical tap and to the second optical tap, for locking the frequency separation based on a ratio of the portions tapped by the optical taps.
 13. The optical communications system of claim 12 wherein the wavelength-locking device comprises: a first sinusoidal generator coupled to a first optical transmitter for injecting a modulation at a frequency f1 onto the optical signal produced by the first optical transmitter; a second sinusoidal generator coupled to a second optical transmitter for injecting a modulation at a frequency f2 onto the optical signal produced by the second optical transmitter; a first photodetector coupled to the first optical tap; a first synchronous detector coupled to the first photodetector and to the sinusoidal generators, for detecting frequency components which are integer multiples of the frequencies f1 and f2; a second photodetector coupled to the second optical tap; a second synchronous detector coupled to the second photodetector and to the sinusoidal generators, for detecting frequency components at the same frequencies as the frequency components detected by the first synchronous detector; and comparison circuitry coupled to the synchronous detectors for comparing a strength of the frequency component at the integer multiple of the frequency f1 detected by the first synchronous detector to that detected by the second synchronous detector, for comparing a strength of the frequency component at the integer multiple of the frequency f2 detected by the first synchronous detector to that detected by the second synchronous detector, and for generating errors signals for the optical transmitters based thereon.
 14. The optical communications system of claim 1 wherein each optical transmitter includes: at least two electrical transmitters for generating electrical channels; an FDM multiplexer coupled to the electrical transmitters for FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and an E/O converter coupled to the FDM multiplexer for converting the electrical high-speed channel into the optical signal.
 15. The optical communications system of claim 14 wherein the at least two optical transmitters comprise: a first optical transmitter for generating a first optical signal containing at least two subbands and a tone, at least one of the subbands including asynchronous I and Q signals.
 16. The optical communications system of claim 15 wherein: each of the asynchronous I and Q signals is based on a separate OC-48 signal; and the subband including the asynchronous I and Q signals has a capacity of approximately 5.0 Gbps of information.
 17. The optical communications system of claim 14 wherein the at least two optical transmitters comprise: a first optical transmitter for generating a first optical signal containing at least two subbands and a tone, each subband having a capacity of approximately 2.5 Gbps of information; and a second optical transmitter for generating a second optical signal containing at least two subbands and a tone, each subband having a capacity of approximately 2.5 Gbps of information, wherein the second optical signal is orthogonally polarized to the first optical signal.
 18. The optical communications system of claim 17 wherein: the first optical signal has a lower optical sideband and an upper optical sideband, each optical sideband containing the at least two subbands and tone; the second optical signal has a lower optical sideband and an upper optical sideband, each optical sideband containing the at least two subbands and tone; and the transmitter subsystem further comprises: an optical filter coupled to the optical combiner for passing the lower optical sideband of the first optical signal and the upper optical sideband of the second optical signal.
 19. The optical communications system of claim 1 further comprising: a receiver subsystem coupled to the transmitter subsystem by an optical fiber for recovering the subbands from the composite optical signal.
 20. The optical communications system of claim 19 wherein the receiver subsystem comprises: a polarizing splitter module for splitting the composite optical signal according to polarization; and a plurality of heterodyne receivers coupled to the polarizing splitter module for recovering the subbands.
 21. The optical communications system of claim 19 wherein the receiver subsystem comprises: an optical splitter module for splitting the composite optical signal; and a plurality of heterodyne receivers coupled to the optical splitter module for recovering the subbands, wherein at least one heterodyne receiver comprises: a polarization controller for matching a polarization of an optical local oscillator signal for the heterodyne receiver and a polarization of a tone in a portion of the composite optical signal received by the heterodyne receiver.
 22. An optical communications system comprising: a transmitter subsystem comprising: a first optical transmitter for generating a first optical signal containing a lower optical sideband and an upper optical sideband; a second optical transmitter for generating a second optical signal containing a lower optical sideband and an upper optical sideband; an optical combiner coupled to the optical transmitters for optically combining the first optical signal and the second optical signal; and an optical filter coupled to the optical combiner for selecting the upper optical sideband of the first optical signal and the lower optical sideband of the second optical signal to produce a composite optical signal.
 23. The optical communications system of claim 22 wherein: at least one of the optical transmitter comprises: a wavelength-tunable optical source, whereby a wavelength of the optical signal generated by the optical transmitter can be tuned by tuning the wavelength-tunable optical source; and the optical filter comprises: a comb filter having periodically spaced pass bands.
 24. The optical communications system of claim 22 wherein the optical filter comprises: two Bragg filters coupled in series.
 25. The optical communications system of claim 22 wherein the optical filter comprises: a comb filter having periodically spaced pass bands.
 26. The optical communications system of claim 22 wherein the optical filter attenuates out-of-band wavelengths.
 27. The optical communications system of claim 22 wherein the transmitter subsystem further comprises: a wavelength-locking device coupled to the optical transmitters for locking a frequency separation of the optical signals to a predetermined value.
 28. The optical communications system of claim 22 wherein each optical transmitter includes: at least two electrical transmitters for generating electrical channels; an FDM multiplexer coupled to the electrical transmitters for FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and an E/O converter coupled to the FDM multiplexer for converting the electrical high-speed channel into the optical signal.
 29. The optical communications system of claim 22 further comprising: a receiver subsystem coupled to the transmitter subsystem by an optical fiber, the receiver subsystem comprising: an optical splitter for splitting the composite optical signals into multiple signals; and a plurality of heterodyne receivers coupled to the optical splitter for recovering information from the signals.
 30. An optical communications system comprising: a transmitter subsystem comprising: an optical transmitter for generating an optical signal containing at least two subbands of information; and a polarization controlling device coupled to the optical transmitter for varying a polarization of the subbands so that the subbands have different polarizations.
 31. The optical communications system of claim 30 wherein the polarization controlling device comprises a birefringent medium.
 32. The optical communications system of claim 30 wherein: the optical transmitter comprises: a wavelength-tunable optical source, whereby a wavelength of the optical signal can be tuned by tuning the wavelength-tunable optical source; and the transmitter subsystem further comprises: a comb filter having periodically spaced pass bands.
 33. The optical communications system of claim 30 wherein: the optical signal has a lower optical sideband and an upper optical sideband; and the transmitter subsystem further comprises: an optical filter coupled to the polarization controlling device for selecting one optical sideband.
 34. The optical communications system of claim 33 wherein the optical filter comprises: a comb filter having periodically spaced pass bands.
 35. The optical communications system of claim 33 wherein the optical filter attenuates out-of-band wavelengths.
 36. The optical communications system of claim 30 wherein the optical transmitter includes: at least two electrical transmitters for generating electrical channels; an FDM multiplexer coupled to the electrical transmitters for FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and an E/O converter coupled to the FDM multiplexer for converting the electrical high-speed channel into the optical signal.
 37. The optical communications system of claim 30 further comprising: a receiver subsystem coupled to the transmitter subsystem by an optical fiber for recovering the subbands from the optical signal.
 38. A method for transmitting information across an optical fiber, the method comprising: generating a first optical signal containing a first subband of information; generating a second optical signal containing a second subband of information, the second optical signal having a different polarization than the first optical signal; optically combining the optical signals into a composite optical signal; and transmitting the composite optical signal across an optical fiber.
 39. The method of claim 38 wherein the optical signals are orthogonally polarized.
 40. The method of claim 38 wherein: each optical signal has a lower optical sideband and an upper optical sideband, wherein an optical sideband of the first optical signal is adjacent to an optical sideband of the second optical signal; and the method further includes the step of optically filtering the optical signals to attenuate the non-adjacent optical sidebands.
 41. The method of claim 40 wherein: the step of optically combining the optical signals into a composite optical signal comprises: optically combining the optical signals so that a lower optical sideband of the first optical signal is adjacent to an upper optical sideband of the second optical signal; and the step of optically filtering the optical signals comprises: optically filtering the optically combined optical signals to select the lower optical sideband of the first optical signal and the upper optical sideband of the second optical signal.
 42. The method of claim 38 further comprising: locking a frequency separation of the optical signals to a predetermined value.
 43. The method of claim 38 wherein each of the steps of generating a first optical signal and generating a second optical signal comprises: generating electrical channels; FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and converting the electrical high-speed channel into the optical signal.
 44. The method of claim 43 wherein: the step of generating a first optical signal comprises: generating a first optical signal containing at least two subbands and a tone, at least one of the subbands including asynchronous I and Q signals.
 45. The method of claim 44 wherein: each of the asynchronous I and Q signals is based on a separate OC-48 signal; and the subband including the asynchronous I and Q signals has a capacity of approximately 5.0 Gbps of information.
 46. The method of claim 43 wherein: the step of generating a first optical signal comprises: generating a first optical signal containing at least two subbands and a tone, each subband having a capacity of approximately 2.5 Gbps of information; and the step of generating a second optical signal comprises: generating a second optical signal containing at least two subbands and a tone, each subband having a capacity of approximately 2.5 Gbps of information, wherein the second optical signal is orthogonally polarized to the first optical signal.
 47. The method of claim 46 wherein: the first optical signal has a lower optical sideband and an upper optical sideband, each optical sideband containing the at least two subbands and tone; the second optical signal has a lower optical sideband and an upper optical sideband, each optical sideband containing the at least two subbands and tone; and the step of optically combining the optical signals into a composite optical signal comprises: optically combining the optical signals so that a lower optical sideband of the first optical signal is adjacent to an upper optical sideband of the second optical signal; and filtering the optically combined optical signals to select the lower optical sideband of the first optical signal and the upper optical sideband of the second optical signal.
 48. The method of claim 38 further comprising: receiving the composite optical signal; splitting the composite optical signal according to polarization; and recovering the subbands using heterodyne detection.
 49. The method of claim 48 wherein the step of recovering the subbands using heterodyne detection comprises: matching a polarization of an optical local oscillator signal with a polarization of a tone in the split composite optical signal; and mixing the pilot tone and the polarization-matched signal.
 50. A method for transmitting information across an optical fiber, the method comprising: generating a first optical signal containing a lower optical sideband and an upper optical sideband; generating a second optical signal containing a lower optical sideband and an upper optical sideband; optically combining the first optical signal and the second optical signal; and optical filtering the optically combined signals to select the upper optical sideband of the first optical signal and the lower optical sideband of the second optical signal to produce a composite optical signal; and transmitting the composite optical signal across an optical fiber.
 51. The method of claim 50 wherein the first optical signal and the second optical signal are orthogonally polarized.
 52. The method of claim 50 further comprising: locking a frequency separation of the optical signals to a predetermined value.
 53. The method of claim 50 wherein each of the steps of generating a first optical signal and generating a second optical signal comprises: generating electrical channels; FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and converting the electrical high-speed channel into the optical signal.
 54. The method of claim 50 further comprising: receiving the composite optical signal; splitting the composite optical signal according to polarization; and recovering the subbands using heterodyne detection.
 55. An method for transmitting information across an optical fiber, the method comprising: generating an optical signal containing at least two subbands of information; varying a polarization of the subbands so that the subbands have different polarizations; and transmitting the optical signal across an optical fiber.
 56. The method of claim 55 wherein: the optical signal has a lower optical sideband and an upper optical sideband; and the method further includes the step of optically filtering the optical signal to select one optical sideband.
 57. The method of claim 55 wherein the step of generating the optical signal comprises: generating electrical channels; FDM multiplexing the electrical channels into an electrical high-speed channel, the electrical high-speed channel further including a tone; and converting the electrical high-speed channel into the optical signal.
 58. The method of claim 55 further comprising: receiving the optical signal; and recovering the subbands using heterodyne detection. 